Modern day multi-storey buildings have generally been considered to require sophisticated air conditioning systems to meet the various needs and requirements of occupants at various locations throughout the building. The peripheral areas of such buildings usually present a varying heating-cooling load at different times during the day and year depending on ambient temperature conditions and solar heat load, whereas the interior core area usually requires cooling the year round. A large part of the heat load at the periphery of the building is due to solar heat gain which can vary significantly during a single day and from day to day, particularly in areas with southern exposures. In view of the different conditions in the peripheral areas and interior core area of multi-storey buildings, it has been common to provide a separate air distribution system for these two areas, providing a constant supply of air to the particular areas at varying temperatures to meet the varying load conditions.
Systems which have been used in the past at the perimeter involve three basic categories; all air, sometimes known as "dual duct" or "terminal reheat" systems; all water, sometimes known as "fan-coil", and air-water; sometimes known as "induction". Systems which have been used in the interior involve principally cool ducted air at constant volume with reheat control.
The majority of prior systems have been arranged with remote fan rooms which supply conditioned air directly to the spaces to be conditioned or to terminal units within such spaces. These systems employ vertical conditioned air duct and return air duct risers and although they allow centralized maintenance of fan equipment, they have introduced a number of problems such as potential recirculation of toxic gases from the scene of a fire to other parts of the building. Deaths can occur when the contamination exceeds 1 percent of the density of smoke at the fire source. Some of these casualties have been traced to smoke spread by fans and vertical duct risers connecting floors.
Other problems with previous systems involving remote fan rooms and vertical duct risers include noise potential due to riser take-off and the use of heavy air pressures in operating the remote fans, and include the loss of space often up to 6 percent of the building gross for air handling equipment and 4 percent for duct shafts. Further, most air conditioned buildings are forced by the nature of the centralized, remote fan room system of handling air to condition the entire building during periods of "after hours" or "odd hours" work of certain tenants on specific floors and this accommodation of the tenants detracts greatly from the operating economy of the system. The majority of systems also depend upon the use of variable outside air ratios for direct free cooling of the building interior. This concept has introduced potential problems such as coil freezing, mixed air stratification heavy dirt loading on filters and the requirement for very large louver areas in the fan rooms.
Further difficulties with typical systems arise at the perimeter of the building where it is conventional to deliver air at the window sill from very bulky enclosures of terminal units such as air-induction or fan coil units. As well as taking up 10 to 30 inches of usable space and detracting from room decor, these enclosures are a source of noise leakage between offices and require regular maintenance within tenant space for filters, nozzles and fan lubrication. Cooling potential of such equipment is often reduced critically by appearance drapes hanging in front of the enclosure.
More recent air conditioning systems have employed a more economical basis of air distribution whereby the supply of cool air is varied in place of reheating a constant volume of cool air in order to maintain temperature control. This system eliminates reheat energy while also reducing energy for cooling and air handling. Such systems are called "Variable Air Volume" (VAV) and it is believed that in their most logical application they are installed with a supply of conditioned cool air along with standing radiation at the perimeter. Such application eliminates the need for separate air handling systems for perimeters and interior space, the single zone Variable Air Volume being sized to absorb all solar load and internal load. The perimeter radiation is sized to neutralize only wall fabric heat loss. The cost for the additional overhead air is a fraction of the premium for the bulky fan coil or induction perimeter.
While a Variable Air Volume system with air handling units arranged on each floor may embody a "static" life safety provision and possible static smoke dispersal control, it generally fails to provide a "dynamic" control through an ability to pressurize vital stair and elevator exit routes from the building. Such a provision is of importance in modern day multi-storey buildings in view of the limitations of normal fire fighting equipment in combatting fires which break out on upper floors of the building and panic caused by smoke dispersal through out the building. Further, a most serious problem with present day air handling units arranged on each floor and which are directly connected to the false ceiling for return air has been muffling the noise of the fan. The provision of a return air duct system above the false ceiling increases the overall costs of a building in necessitating a deeper false ceiling plenum to permit cross-overs between the duct work. Sound control can be added at the boundary of a fan utility room but this correction must rely on field workmanship to such an extent as to be impractical when trying to meet the usual criteria of NC-40 alongside the fan room and NC-35 in the perimeter area or executive office space area on a floor. On-floor fan units of the size necessary to deal with typical office floors of 10,000 sq. ft. or more can not be silenced with simple field applied sound silencers in the inlet and outlet ducts. Even the casing of such equipment emanates enough noise to exceed the usual noise criterion noted above.
Present day HVAC systems often have sophisticated and complicated automatic control systems, the complexities of which increase as the need for local control arises. Often the design of such control systems falls beyond the comprehension of the average building system operator with resulting peril for the equipment and the beneficiaries of the system. Moreover, such control systems have initial high cost and require regular inspection and maintenance to maintain the HVAC system functional.